All eukaryotic cells share the ability to sense and respond to changes in their environment. The plant hormone, abscisic acid (ABA), plays a critical role in regulation of stress responses in plants. Protein phosphorylation is an important mechanism for ABA-mediated processes. Although several protein kinases implicated in ABA signaling have been identified, their target proteins remain unknown. T-DNA insertion mutants of the ABA-activated protein kinase SnRK2-2 exhibit ABA-dependent phenotype. Furthermore, two SnRK2-2-dependent phosphoproteins have been detected. Based on these studies, we hypothesize that these two phosphoproteins are the substrates of the SnRK2-2 kinase and mediate AE3A response. The main goal of the proposed research is to identify two SnRK2-2-dependent phosphoproteins and to determine their roles n ABA-mediated responses. Our research plan has three specific aims: (1) identify these two phosphoproteins by tandem mass spectrometry; (2) analyze phosphorylation of the two proteins by active SnRK2-2 kinase; (3) determine the roles of these two phosphoproteins in ABA-mediated responses by reverse genetic approaches. Because a protein kinase regulates biological processes through modulation of its target proteins, identification of kinase substrates will provide significant insight into the function of that kinase in cells. SnRK2-2 is an SNF1-related protein kinase. The SNF1/AMP-activated protein kinase (AMPK) family plays fundamental roles in cellular responses to metabolic stress in eukaryotes. In humans, AMPK has been implicated in metabolic disorders including diabetes, obesity, and cardiac abnormalities. By learning more about substrate proteins we hope to better understand the function of SnRK2-2 in ABA-mediated stress response and to shed more light on the function of SNF1-related protein kinases in other eukaryotic organisms including humans.